Method of making high temperatureresisting bonded mica products



Oct. 23, 1945.

w. A. BOUGHTON ET AL 2,387,559 METHOD OF MAKING HIGH TEMPERATURE-RESISTING BONDED MICA PRODUCTS Filed Nov. 20, 1942 Patented Oct. 23, 1945 METHOD f TED STATES PATENT OFFICE g I 2,387,559 v RESISTING BONDED MICA PRODUCTS Patents Trust, Cambridge, Application November 20, 1942, Serial No. 166,255

'IClaims.

This invention relates to a method of making a high temperature resisting bonded mica product.

The invention has for an object to provide a novel, improved and practical method of manufacturing high temperature resisting inorganic bonded mica products by which a product of greater uniformity, mechanical strength, and integration may be economically produced.

With this general object in view, and such others as may hereinafter appear, the invention consists in the method of manufacturing mica products herein described and particularly defined in the claims at the end of this specification.

Mass.

In the drawing, Fig. 1 is a transverse sectional view illustrating a typical heater charge .comprising a single unbaked and uncompressed but superficially dried mica plate, with adjacent prefabricated members covering the top and bottom thereof, ready for processing in accordance with the present invention; and Fig. 2 is a similar view illustrating a heater charge comprising a plurality of unbaked and uncompressed but partially dried mica plates separated by prefabricated members.

In general, the present invention contemplates an improvement upon the method disclosed in the also applicable to the present method to be hereinafter described. As therein disclosed, the preferred method of constructing the laminated plate comprises scattering a layer of mica film on a screen and covering the layer with a thin layer of sprinkled, sprayed or painted inorganic binder liquid in controlled quantities. Additional layer of mica films are added with binder liquid between them until built up to approximately the desired'thickness. The other half of the plate is built similarly on another screen. One of the halves is then inverted and the other half is lifted and placed on the inverted half with an application of binder liquid between. The completely built green plate is then placed onto another screen and superficially dehydrated by heating to a suitable temperature and under'such conditions as to remove substantially all of the .solvent from the binder, as described in the patent above re- United-States Patent No. 2,186,954, dated January 16, 1940, to Willis A. Boughton and William R. Mansfield, in which a plurality of inorganic bonded and superficially dried mica plates are assembled into a pile with the unbaked and uncompressed mica plates separated from each other by metallic sheets preferably having a low or negligible coefiicient of expansion, the assembly being provided with suitable bottom, intermediate and top metal plates, to form a charge to be placed in a heating furnace. In accordance with the prior method the heater charge so assembled is placed in the heating furnace and heated to a red heat, whereupon the red hot charge, with the binder in a molten viscous state, is placed in a suitable press, compressed, andthen permitted to cool under compression.

The inorganic binder for such high temperature I resisting mica plate may comprise any of the known operative inorganic compounds, associations or mixtures thereof, capable of eflicient use as binders for high temperature resisting mica plate such as the mixtures of monosodium phos-' phate and borax described by Boughton and Mansfield in Patent No.2,186,9 54,

The method of constr c i is't e laminated mica plate and for superficially dehydrating the green or unbaked plate described in said patent, as well as formerly used methods for these operations, are

ferred to.

In accordance with the prior method, the superficially dehydrated green plate was then built up with others of similar construction into a baking charge comprising a plurality of green mica plates separated from one another by thin metallic separator sheets, the top and bottom of the assembly being provided with heavier metal plates and the several courses being also separated by heavier metal plates, as described in the above patent. The multiple charge, comprising the assembly of superficially dried green plates, was then placed in a suitable furnace which was closed and heat applied, without the addition of further pressure upon the plates than that due to the weight of the material forming the charge, until a binder fusion temperature of red heat was reached, for example about 550 C. to about 650 C. for the inorganic binder specifically described therein. The charge, while still red hot with the binder in a molten viscous state, was drawn into a suitable press and immediately a pressure of from about 250 to about 500 pounds per square inch is applied. The mica films were thus bonded to form a plurality of integral plates. Upon reaching this stage, the

entire charge was cooled preferably by water cooled platens or by air blasts, while the assembly was still under pressure.

In the above referred to patent, Boughton and Mansfield stated that the metal sheets, which served to separate the mica plates in the heater charge, should have a low or negligible thermal coefiicient of expansion at all temperatures, that is, of the same order as that of the mica plates themselves, for example a metal of the Invar type, and they cited specifically a nickel-steel known as Allegheny electric metal" containing 47% of nickel. They defined approximate equiv-' alence of the thermal coemcients of expansion of the mica plate and. th metal separator sheets during both heating and cooling operations as the criterion of operability of the metal separator sheet.

The importance of maximum equivalence of the thermal coefficient of. expansion of the mica plate and the metal separator sheets at all times and at all points during the heating and cooling operations resides in the fact, established in extensive production tests, that any differences in the respective expansion coefilcients caus differences in degree of compression upon different areas of the mica plate during cooling of the heatercharge under compression, with consequent differences in mechanical integration of various parts of the mica plate.

The differences in thermal coefficients of expansion between the mica. plates and metal separator sheets exhibit also, as the result of difierences in rates of contraction of the mica plates and metal sheets upon cooling from temperatures of redness downwards, the efiect of buckling or wrinkling of the mica plates at certain points during the cooling operation. 7

In accordance with the present invention, the

foregoing method of making high temperature resisting mica plate, and particularly the steps of heating an assemblyof the plate or plates to red heat and of subsequently cooling the same under pressure, have been modified with respect to the use in the heater charge between the adjacent mica plates, of non-metallic separator plates having substantially the expansion and ing manufacturing operations, and in the case of the charge containing a plurality of green plates, we may use base, top, and intermediate steel plates, as described by Boughton and Mansfield, with the non-metallic separator plates between adjacent mica plates. 7

We haveestablished that the use of pre-fabricated mica plates having greater thickness than .the green plates produces improved results, al-

though it is not desired to limit the invention in this respect.

Wehave found also that thin layers of finely powdered inorganic fillers, such as powdered mica or talc, between the green" and pre-fabricatedf mica plates are desirable, to assist ease of dissembly of the processed charge, but again it is other non-metallic separator plates or members have been used successfully, including asbestos sheets, mica sheets and unbonded mica layers.

In a co-pending application filed on even date herewith, however, Boughton, Mansfield and Hughes have shown that differentials between the rates of thermal contraction between the outer areas toward the edges and the inner areas toward the center of the mica plate in the heater charge containing a plurality of mica plates due to temperature diilerences in the heater charge during cooling under compression constitute a deleterious factor in the manufacture of composite inorganic-bonded mica plate. This is separate and distinct from and additional to the deleterious effects caused by differences in equivalence of the thermal coeflicients of expansion tion and, more important, during the cooling operation, and by virtue of this equivalence the deleterious effects of any differences in rates or degrees of thermal expansion and contraction between the mica plate being processed and the separating medium are eliminated.

The pre-fabricated inorganic-bonded mica plate I! to be used as the separator medium may be made by the method described by Boughton and Mansfield, with metallic sheets serving as separators in the heater charge assembly, or it may be made as a'single sheet, with metallic sheets placed on each side comprising the heater charge, as described by Boughton, Mansfield and Hughes in a co-pending application filed on even date herewith.

In practicing the present method, we may assemble a heater charge comprising one green mica plate III with prefabricated inorganicbonded mica plates I! as top and bottom covers,

. as illustrated in Fig. 1, or a charge comprising a pluralit of green mica plates I II separated from each other by pre-fabricated inorganic-bonded micaplate's II, as illustrated in Fig. 2. In the and contraction of the mica plates and the metal, separator sheets during the heating and cooling to differences in equivalence to the thermal coefficients of expansion and contraction of the mica plates and separator sheets.

Having thus described the invention, what is claimed is;

1. The method of making a high temperature resisting composite mica product which comprises building up mica films into a plate by cementinga plurality of layers of said films together with a high' temperature resisting inorganic binder, said binder being applied as a liquid, removing the solvent from said binder within the plate, placing said superficially dried mica plate between prefabricated high temperature resisting composite inorganic bonded mica plates to form an assembly, baking said assembly in a furnace at a binder fusion temperature of from about 550 C. to about 650 C., compressing said assembly while still hot under a pressure of from about 250 to 500 pounds per square inch, and cooling said assembly while still under pressure.

The method of making high temperature resisting composite mica products which comprises constructing a plate from a plurality of layers of mica films. cemented. together with a high temperature resisting inorganic binder, su-

perficially dehydrating the binder in said plate, piling aplurality of said plates so treated to form a heater charge, each 'plate being separated from the other by at least one prefabricated high temperature resisting composite inorganic bonded mica plate, baking said charge in a furnace at a temperature that will fuse the binder, compressing said charge while still hot, and permitting said charge to cool while still under comresisting laminated inorganic-bonded mica plate,

the steps comprising heating the plate being processed between separator sheets comprising prefabricated mica plates bonded with the same inorganic binder as the plate being processed,

and subsequently cooling the thus heated plate under compression whereby to obtain maximum equivalence of thermal coeflicients of expansion between the mica plate being processed and the separator sheets.

5. In themethod of making high temperature resisting laminated inorganic-bonded mica plate,

the steps comprising heating the plate being 35 processed between mica separator means and subsequently cooling the thus heated assembly while maintaining the plate under compression.

6. The method of making a high temperature resisting composite mica product which comprises building up mica films into a plate by cementing a pluralityof layers of said films together with a high temperature resisting inorganic binder, superficially dehydrating the binder in said plate,

. placing the superficially dried mica plate between prefabricated mica plates having a thermal coeilicient of expansion substantially equal to the mica plate being processed, baking. theassembly in a furnace at a temperature that will flux the binder, compressing the assembly while still hot. and permitting the assembly to cool while still under compression, said equivalence of thermal coefficients of expansion operating to reduce to a minimum the deleterious stresses in the plate being processed during the heating and cooling operations.

7. In the method of making a high temperature resisting int-egratedmica plate bonded with an inorganic binder, the steps of placing a superficially dried green plate between separating members comprising prefabricated mica plates characterized by a thermal coeilicient of expansion of the same order as that of the inorganic bonded mica plate, baking the assembly in a furnace at a temperature that will flux the binder in the green plate, compressing the assembly while still hot, and permitting the assembly to cool while still under compression, whereby the deleterious eflfects of any difierences in degree of thermal expansion and contraction between the. green plate and the separating medium are reduced to a minimum; Y

WILLIS A. BOUGH'I'ON. 

